This invention relates to diversity combining systems and, more particularly, to a large dynamic range multiplier for maximal-ratio diversity combiners.
The need for space diversity combining arises in mobile radio systems because the radio-frequency (RF) signal path between a mobile transmitter and a base receiver is generally not line of sight, but instead consists of many reflected and scattered RF signal paths having varying amplitudes and phases. Furthermore, in mobile radio systems operating at relatively high frequencies, for example at 800 MHz, deep, rapid fading, commonly referred to as Rayleigh fading, must be contended with. By utilizing an antenna array having space diversity, the foregoing effects may be substantially reduced. According to space diversity, antennas of an antenna array are spaced at predetermined distances from one another, for example, at a distance of at least one-quarter wavelength from one another. The probability that deep fades will occur simultaneously at all antennas of a space-diversity antenna array will be extremely low. Thus, a composite signal formed by coherently combining each of the RF signals from a space-diversity antenna array will theoretically have a signal level at least as high as the strongest RF signal received by the antenna array.
One practical prior art technique of coherently combining the antenna RF signals from a space-diversity antenna array is known as "equal-gain predetection diversity combining". Exemplary equal-gain predetection diversity combiners are those described in an article by D. Brennan entitled, "Linear Diversity Combining Techniques", published by IRE Proceedinngs, June 1959, at pp. 1075 to 1101 and in U.S. Pat. No. 3,471,788 to W. S. Bickford et al. In these prior art combiners, the antenna signals are converted to intermediate frequency (IF) signals which are then cophased with one another and thereafter linearly combined to provide a composite IF signal. For example, the IF signals developed from each antenna RF signal may be phase aligned with a locally generated signal of a reference frequency, or may be phase aligned to a selected one of the IF signals, or may be phase aligned with respect to the composite IF signal. Once the IF signals from each antenna RF signal are cophased with one another, they may then be linearly added by appropriate circuitry to provide a coherent composite IF signal which is the vector sum of the individual IF signals.
In order to cophase each IF signal, prior art equal-gain predetection diversity combiners include circuitry, commonly referred to as a "branch", for dividing the IF signal into first and second portions, mixing the first portion with a reference signal to provide a first product signal that has a phase equal to the difference in phase between the first portion and the reference signal, and mixing the first product signal with the second portion of the IF signal to provide a second product signal that will be cophased with the reference signal. Since the second product signals of each branch are cophased with one another, they may then be linearly added by appropriate circuitry to provide the composite coherent IF signal. If the second portion of the IF signal and the first product signal were each amplified linearly so that the magnitude of each would be proportional to the input IF signal, the magnitude of the second product signal would theoretically be proportional to the square of the magnitude of the input IF signal. However, in the conceptual design of the prior art equal-gain combiner, the first product signal is amplitude limited prior to the input to the second stage of mixing. Consequently, the second product signal will be directly proportional to the magnitude of the input IF signal rather than to its square.
In such equal-gain combining systems, it is necessary that all of the antenna RF signals must have substantially the same mean signal level due to the fact that the first product signal is amplitude limited prior to the second mixing. Because the first product signal is amplitude limited, the second product signal from the second stage of mixing will not be proportional to the square of the magnitude of the input IF signal. Thus, if IF signals received by all branches do not have substantially the same mean signal level, the composite IF signal may be significantly degraded in signal-to-noise ratio since a weak signal received by one branch will be weighted substantially equally with a strong signal received by another branch.
The foregoing inadequacy of prior art equal-gain predetection diversity combiners may be improved by utilizing the prior art combining technique known as "maximal-ratio predetection combining". In such maximal-ratio predetection combining systems, signals are not limited prior to the second mixing in each of the branches. It is desired that all signals are proportionally related to the input IF signal so that the magnitude of the second product signal will be proportional to the square of the magnitude of the input IF signal. As a result, branches receiving strong signals will receive more emphasis than branches receiving weak signals.
Since Rayleigh fading experienced in 800 MHz systems may cause instantaneous amplitude variations between the RF signals received at different antennas of a space-diversty antenna array that are in excess of 40 decibels (db), the linear dynamic range of the branch circuitry must accommodate IF signals having at least a 40 db dynamic range and second product signals having amplitude variations in excess of 80 db, which is twice the dynamic range of the input IF signals due to the squaring by the second mixing operation. Thus, the particular circuit implementation for providing the second mixing operation must provide substantially idealized multiplication over an extremely large dynamic range.
A prior art product multiplier capable of providing the desired performance over an output dynamic range in excess of 80 db has not been practically achieved in the past. Commercially available linear integrated-circuit balanced mixers such as the Motorola MC1596 have been utilized as the second mixer in maximal ratio combiners designed for military applications. These integrated circuit mixers consist of a quad differential amplifier with cross-coupled outputs to provide full-wave balanced multiplication of the two input signals. Each differential pair is powered by a constant current source. Such a mixer will not accommodate input signals having a dynamic range in excess of 30 db, since its linear output dynamic range is only 50 to 60 db. A doubly-balanced FET mixer, such as that described by Highleyman and Jacob in the article, "An Analog Multiplier Using Two Field Effect Transistors", IRE Transactions on Communications Systems, Vol. CS-10, pp. 311-317, September, 1962, may also be used for the second mixing operation with some expected improvement in dynamic range, but without any appreciable reduction in complexity or cost.
Accordingly, it is an object of the present invention to provide an improved low-cost, large dynamic range product multiplier for a maximal-ratio predetection diversity combiner that coherently combines a plurality of amplitude and phase varying input signals of substantially the same frequency.
It is another object of the present invention to provide an improved low-cost, large dynamic range product multiplier for a maximal-ratio predetection diversity combiner suitable for use in a diversity receiver for coherently combining RF signals having a dynamic range in excess of forty decibels (40 dB).